Here is the full list of all 37 Psychonauts achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Achieve Rank 80.
Complete the Punchy Target mini-game.
Buy an item from the Camp Store on Christmas.
See yourself through the eyes of many others.
Give the Coach's speech on the stump.
Discover the secret history of Whispering Rock.
Use the bacon. A lot.
Read many bulletin board messages.
Introduce all Camp Kids to Mr. Pokeylope.
Uncover Milla's Secret.
Re-brain the Children.
Redeem all 16 Scavenger Hunt Items.
Achieve Rank 60.
Complete Black Velvetopia.
Find all Golden Helmets.
Earn all Merit Badges.
Find Edgar's Secret Garden.
Complete Waterloo World.
Complete Gloria’s Theater.
Complete The Milkman Conspiracy.
Achieve Rank 40.
Complete Lungfishopolis.
Spy on Bobby's love life.
Hear Vernon's Ghost Story.
Complete Milla’s Dance Party.
Achieve Rank 20.
Complete Sasha’s Shooting Gallery.
Witness Maloof's transformation.
Complete Basic Braining.
Complete The Asylum.
Complete Meat Circus.
Achieve Rank 101.
Sort all Emotional Baggage.
Crack all Vaults.
Gather all Figments.
Revisit all brains after completion.
Cook and consume two different kinds of roast in one sitting.